Sir Alex Ferguson has ended his media silence by accepting Manchester United have no excuses for their performance at Liverpool and admitting that the defeat leaves the second-placed Arsenal as an increasingly serious danger to his hopes of returning the Premier League trophy to Old Trafford.

Ferguson did not comment about the controversies surrounding United's third successive league defeat at Anfield, most notably the tackle by Jamie Carragher that split open Nani's left leg, and he refrained from discussing his reasons for imposing a media blackout after the game.

However, the United manager, preparing to face Arsenal in an FA Cup quarter-final at Old Trafford on Saturday, was clearly aggrieved by the team's performance, with three defeats in their past five league games representing their worst run since 2004. "I have no excuses for the poor performance at Liverpool, who were the better team and deserved their win," Ferguson said. "I was disappointed because we didn't perform to the level I expect and which, to be fair, we generally achieve."

The defeat means Arsenal can pull level on points with United at the top of the table if they win their game in hand, and Ferguson expressed mild surprise about how Arsène Wenger's side have emerged as United's most credible challengers for the title. Ferguson had been prepared for Chelsea to be the most significant threat but it is Arsenal, despite their defeat to Barcelona in the Champions League, who are troubling him the most.

"Last season they faded around the February to March period to leave Chelsea and ourselves in a two-horse race that saw us pipped at the post by a point. I think most pundits expected to see something similar this season, plus a challenge from Spurs and Manchester City to make it even more difficult for Arsenal to get back up to the top.

"But Arsène Wenger and his players have rearranged the pecking order somewhat to come storming through to lie in second place. Clearly they represent the main danger to us achieving our ambitions in a revival of the rivalry of a few years ago, and I am sure they regard us as the major threat to their hopes."

Ferguson's preparations for the game on Saturday have been hampered by Nani's injury, with the Portuguese winger expected to be out for up to a fortnight. Antonio Valencia played for an hour in a behind-closed-doors match this week and could be included in the squad for the first time since breaking his leg last September, but United chose against him including him in their reserve match at Blackpool on Wednesday because of the poor state of the pitch at Bloomfield Road. Park Ji-sung may also be involved after his hamstring strain.

After losing two successive games playing with a 4-4-2 system, Ferguson is considering reverting to the 4-5-1 formation he usually favours when United play their main rivals, and that helped the team to a 1-0 defeat of Arsenal in December. Dimitar Berbatov missed out on that occasion, which has become a recurring theme for the Bulgarian despite his status as the club's leading scorer, but his worth to the team is expected to be rewarded shortly with a 12-month extension to his contract.

When Berbatov signed from Tottenham Hotspur in September 2009 United inserted a "one-way option" clause into his four-year contract that meant the club could extend the deal to the end of the 2012‑13 season, as long as they made that decision before the end of the current season.